1. Field of the Invention
The present invention relates to an ejection device that ejects droplets of liquid, and more specifically to an ejection device capable of precisely ejecting droplets at high speed in desired resolutions.
2. Related Art
Japanese Patent-Application Publication No. HEI-11-78013 discloses an inkjet recording device, which is one example of droplet ejection devices. Such an inkjet recording device includes an elongated inkjet recording head formed with a plurality of nozzles aligned equidistance from each other. The nozzle line is angled with respect to a sheet feed direction in which a recording medium is transported. When an energy generating element of each nozzle is applied with a driving voltage based on a recording signal, then a pressure is applied to ink inside an ink chamber, thereby an ink droplet is ejected through an orifice. Thus ejected ink droplet reaches the recording medium and forms a recording dot thereon. Recording operations are performed in this manner. This type of inkjet recording device has a simple configuration and is capable of high speed printing.
FIG. 1(a) shows a piezoelectric-element driver 1420, which is one example of conventional piezoelectric-element drivers, connected to 128-number of piezoelectric elements 304. A common power source 202 is connected to a common terminal 304b of each piezoelectric element 304 for supplying a 40V direct current to the piezoelectric elements 304 which could be driven by at least 10V electric current. The piezoelectric-element driver 1420 includes 128-number of switches 1203 connected to the corresponding 128-number of piezoelectric elements 304, a 128-bit latch 204, a 128-bit shift register 205, and a rectangular-waveform generating circuit 1206. A binary ejection signal 207 is input to the shift register 205 and shifts one bit at a time in synchronization with the shift-clock S-CLK. The ejection signal 207 having a value xe2x80x9c1xe2x80x9d indicates xe2x80x9cejectionxe2x80x9d, and the ejection signal 207 having a value xe2x80x9c0xe2x80x9d indicates xe2x80x9cnon-ejectionxe2x80x9d. The latch 204 latches 128-bit data from the shift register 205 in synchronization with a pixel-synchronization signal 109 (latch clock L-CLK). The rectangular-waveform generating circuit 1206 generates a common output-enable (OE) signal 206 having a predetermined width in synchronization with the latch clock L-CLK. A logical product of an output from the latch 204 and the common OE signal 206 is input to a switching terminal of each switch 1203. The switch 1203 connects the individual terminal 304a of the piezoelectric element 304 to the ground when a value xe2x80x9c1xe2x80x9d is applied to the switch terminal, so that a driving waveform Vpzt shown in FIG. 1(b) is applied to the piezoelectric element 304. On the other hand, the switch 1203 connects the individual terminal 304a to the common power source 202 when a value xe2x80x9c0xe2x80x9d is applied, so that no driving waveform Vpzt is applied to the piezoelectric element 304.
An example of operations of the piezoelectric-element driver 1420 will be described with reference to the timing chart of FIG. 1(b). In this example, the common OE signal 206 is a well-known rectangular waveform having a driving voltage of 40V and a time-width of 5 xcexcm to 25 xcexcm. When the pixel-synchronization signal 109 is received, then the pixel-synchronization signal 109 is input as the latch clock L-CLK to the latch 204 so that the ejection signals 207 that have been stored in the shift register 205 in a previous cycle are stored in the latch 204 at once. Then, the common OE signal 206 that is generated in synchronization with the pixel-synchronization signal 109 is input to the AND circuit. As a result, nozzles whose ejection signals 207 have the value of xe2x80x9c1xe2x80x9d eject ink droplets, and nozzles whose ejection signals 207 have the value of xe2x80x9c0xe2x80x9d eject no ink droplets. Then, subsequent ejection signals 207 are input to the shift register 205 in synchronization with the shift-clock S-CLK, and the process waits until the next pixel-synchronization signal 109 is generated.
There have been also provided piezoelectric-element drivers having different configurations. However, these drivers are common in applying an analog voltage to the common terminals of the piezoelectric elements and in switching the connection at the individual terminals. This type of piezoelectric-element driver has a simple configuration and is particularly indispensable in multi-nozzle inkjet recording devices.
Here, in order to form high-quality half toning images like photographical images, multiple level halftoning that creates the appearance of intermediate tones of black, white, and a plurality of gray levels is necessary. There have been known two methods for realizing such multiple tone levels. The one is to control a number of recording dots in a single pixel area, and the other is to change a mass of each droplet by controlling a corresponding driving waveform Vpzt. The latter method is known to be preferable in highly-reliable high-speed inkjet recording devices.
It is conceivable to control an individual driving waveforms Vpzt by providing an individual driving circuit for each one of the nozzles. However, it is not practical to provide so many driving circuits in a multi-nozzle inkjet recording device that includes a great number of nozzles since it greatly increases manufacturing costs of the device. Moreover, in a conventional piezoelectric-element driver such as those shown in FIG. 1(a), it is necessary to change the analog voltage from the power source 202 each time for each nozzle in order to change the driving waveform Vpzt. However, it is difficult to change the analog voltage in such a manner.
A recording resolution is determined by a nozzle density. For example, if the nozzle density is 300 nozzles per inch (npi), then the recording resolution is usually 300 dots per inch (dpi). In order to form a 240 dpi image using a recording device having the nozzle density of 300 dpi, a well-known digital data process, such as enlargement process, high-resolution process, or the like is previously performed to obtain converted data, and then the recording is performed based on thus obtained data.
However, it is preferable to avoid such a digital data process since the process usually changes or degrades image quality, disabling to provide images desired by users.
In view of forgoing, therefore, it is an object of the present invention to overcome the above problems and also to provide a high-speed ejection device having an elongated head capable of ejecting droplets on precise locations in a designated resolution.
It is also an object of the present invention to provide a multi-nozzle inkjet recording device capable of stably forming high-quality multi-toning images by changing a mass of each ink droplet.
In order to achieve the above and other objects, according to the present invention, there is provided an ejection device including a head formed with a plurality of nozzles arranged in a row for selectively ejecting droplets from the nozzles so as to form dots onto a medium, a transporting means for transporting the medium relative to the head in a first direction, a resolution specifying means for specifying a resolution with respect to the first direction, a preciseness specifying means for specifying preciseness in dot locations on the medium, an angle specifying means for specifying an angle of the head with respect to a second direction perpendicular to the first direction based on the specified resolution, a sub-pixel determining means for determining a size of a sub-pixel with respect to the first direction based on the specified preciseness, a converting means for converting an ejection data to a sub-pixel data based both on the specified resolution and the size of the sub-pixel, and a control means for controlling the head based on the sub-pixel data to selectively ejecting the droplets from the nozzles.
There is also provided an ejection device including a head formed with a plurality of nozzles arranged in a row that is angled with respect to a first direction, a transporting means for transporting a medium with respect to the head in a second direction perpendicular to the first direction, a timing-signal generating means for generating a timing signal in accordance with a position of the medium, a driving-signal generating means for generating a driving signal in synchronization with the timing signal, a converting means for converting an ejection-tone data into a pulse-width signal in synchronization with the timing signal, a chance-signal providing means for providing a chance signal that provides a chance for ejection to a selected one of the nozzles at a time in synchronization with the timing signal, and a control means for controlling the head to selectively eject a droplet from the selected nozzle based on the driving signal, on the pulse-width signal, and on the chance signal.